Stamping and/or blanking processes are traditionally used to produce a plurality of profiled blanks.
For example, it is known that some types of electric motors are produced by stacking a plurality of metal laminations. In particular, the stators and rotors of said motors are produced by packing a plurality of appropriately profiled ferromagnetic laminations. Generally the individual laminations are obtained from metal blanks which undergo stamping and blanking processes to obtain the required shape. The laminations thus obtained are coupled, in particular stacked, to form the core of a rotor or to form a stator.
The stamping and/or blanking processes are also used to produce other types of objects such as, for example, small gears, blanked pieces, computer cases, profiled parts etc.
Normally the stamping and blanking of blanks, also semi-finished products, are obtained by means of specific tools. The tools are provided with a die which, cooperating with a punch, performs the stamping of the blanks fed to the tool, or the blanking and the separation of the laminations. The punch is connected to a portion of the tool which moves vertically and alternatively on the blank, which remains positioned between the punch and the die. The forward movement of the blank is coordinated with the movement of the punch, so that each time the punch is lowered, new portions of the blank are intercepted by the punch and the die in order to be stamped or blanked. A tool for the production of metal laminations for electric motors is described, for example, in the European patent application EP-A-1859876 in the name of the Applicant.
Traditionally, the portion of the tool that supports the die is the lower stationary portion, while the portion of the tool that supports the punch is the upper portion, which is moved vertically with reciprocating motion. The upper portion of the tool is appropriately guided in its vertical movement so that the punch and the die are always correctly aligned.
The tools are generally provided with a device for guiding the upper portion of the tool with respect to the lower portion. The guide device comprises two or more “pillars”, generally rigid rods connected to the upper (or lower) portion of the tool, which engage the lower (or upper) portion of the tool in a sliding manner.
When the punch is lowered to perform blanking of a portion of the blank, the pillar also moves vertically, engaging a distal end thereof with a seat purposely obtained in the lower portion of the tool, opposite the punch. In this way the guide device keeps the punch and the die centred during the stamping or blanking step. On the outer surface of the pillars, a plurality of revolving metal spheres can be provided which transform the sliding friction into rolling friction when the pillar fits into the corresponding seat obtained in the other portion of the tool.
The current tools can operate at high speed. For example the punch can be operated 300 times per minute. The precision of the guide device in aligning the two portions of the tool (upper and lower) is important to guarantee high quality and production standards.
In addition to the locating pillars, designed to guide the portions of the tool with respect to each other, modern tools are provided with locating devices to center the blank with respect to the die and relative punch. In other words, a locating device, or more simply “locator”, has the task of correctly aligning the blank on the plane to ensure precise positioning with respect to the die and the punch.
Modern tools are also provided with a stripper element, i.e. a moving element, automatically operated, which has the task of facilitating separation of the blanked portion from the remaining portion of the same blank. The stripper element, which is different and separate from the locating devices, can have different forms, for example it can be generically cylindrical or tapered, and is connected to the upper portion of the tool, i.e. to the same portion where the punch is housed. The stripper moves with respect to the lower portion of the tool, on which the die is mounted, but it can also move with respect to the upper portion of the tool by means of appropriate mechanical actuators. The movement of the stripper element is synchronised with the movement of the upper portion of the tool.
U.S. Pat. No. 6,163,949 discloses in a detailed way the various steps of stamping and blanking of a blank for obtaining the laminations of stator and rotor of an electric motor. The positioning of the blank during its movement in the tool is performed by means of a plurality of circular driving holes in preset positions. In each stamping or blanking station of the tool, a circular pin, with a slight conical end, is inserted with interference in to the centring hole. In order to avoid that extraction of the pin, at the end of a stamping or blanking step, would raise the blank and therefore prevent the advancing thereof, a spring can be provided for exerting pressure on the same blank. The operation of a traditional tool during a metal lamination blanking cycle will now be described.
When the tool is open, i.e. when the upper portion is raised with respect to the lower portion, the blank is moved forward between the two above-mentioned portions. When the area of the blank to be blanked corresponds to the die, the upper portion of the tool is lowered towards the lower portion. Before the punch interacts with the relative die to blank a portion of the blank, the locator and the stripper element simultaneously engage with the blank to lock it in position, preventing lateral movements thereof. In this configuration the stripper acts as a retaining element for the blank.
Normally the locator has a cylindrical form, and the end that engages the sheet (a hole in the sheet) is slightly conical or flared on the lateral surface. Said end fits into a hole on the sheet undergoing the work process.
The blanking of the blank is completed by the punch which penetrates partially into the die. In a subsequent step, in which the tool is opened, the upper portion, the punch and the locator are raised with respect to the lower portion and the die, while the stripper remains lowered, for example in contact with the blanked portion or the remaining part of the blank. This prevents lifting of the blanked portion and/or the blank, and therefore facilitates separation of the locator with respect to the blank in order to allow the immediate advancing thereof.
Indeed, in order to allow a high accuracy of positioning, the locator has a dimension such as to engage the centring hole with a certain interference. If the stripper were not present, the blank could be raised together with the upper portion of the tool by the mechanical interferences that can occur between the locator and the centring hole in the blank undergoing the work process. When the upper portion of the tool has covered a pre-defined stroke in the lifting movement, the stripper is also raised together with it to re-set the tool to the open position, ready for a new blanking cycle. In traditional tools not provided with stripper, in order to avoid mechanical interference between the locator and the blank, the locator has a cylindrical form, normally with diameter smaller than the diameter of the centring hole. This solution prevents perfect centring of the blank since no mechanical interference is created between the locator and the centring hole of the blank.
Also in traditional tools provided with stripper, the locator does not permit high-precision positioning of the blank, i.e. centring of the blank is not always optimal. Indeed, positioning of the blank in each working station is always performed in the same hole and, due to the mechanical interference, deformations could take place in such a way that would cause low accuracy positioning because the reference is no longer univocal, in particular at the final working stations.
The partial insertion of a conical locator element in a circular centring hole which is remote from the portion undergoing to stamping and/or blanking could also cause undesired movements of the blank before reaching the correct centring.
Moreover, positioning of the blank moving from a station to another of the tool is not always optimal between the two upper and lower portions of the tool and intervention of the locator is of no help, as the lamination is retained by the stripper before the locator could perform the correct positioning of the blank.